This invention relates to the design and construction of electromagnets. More particularly, the invention features a cylindrical electromagnet useful as a gradient coil for providing spatial information in nuclear magnetic resonance ("NMR") imaging systems.
Recent years have evidenced an explosion in technological advances related to health care. The computer-aided tomographic ("CAT") scanner provides a new tool for the medical profession to explore the interior of the human body without surgery. The CAT scanner uses x-rays as a probe and produces an absorption pattern which a computer reconstructs to form an image. While the CAT scanner has changed diagnostic medicine by allowing non-invasive in-depth exploration of the body, the exposure of patients to x-rays should be limited because of the potential damage caused by this high energy form of radiation. Therefore, less dangerous alternatives have been sought to replace the CAT scanner.
The search for safer alternatives to the CAT scanner led to the development of specialized NMR imaging systems for biomedical applications. The basic NMR imaging system for biomedical applications includes a bias field magnet, a series of gradient coils, an RF or radio frequency excitation magnet, a sequencer, and a receiver. NMR imaging systems are based on the interaction between the magnetic moments of nuclei, and an applied magnetic field. Currently, hydrogen nuclei are used in most NMR imaging systems but other nuclei having a net nuclear magnetic moment, e.g., phosphorus nuclei, could be used to provide additional information. The bias field magnet produces a substantially constant, preferably uniform, magnetic field, causing a shift in the direction of the magnetic moments of the hydrogen nuclei so more magnetic moments are aligned parallel to the applied field than antiparallel. The RF magnet then is turned on to produce a pulse of radio frequency fields, causing a short term perturbation of the magnetic field. The nuclei absorb energy at a frequency in this range so the magnetic moments of the nuclei are displaced from their predominately aligned orientation to a non-aligned position. This non-aligned position is a higher energy state than the aligned orientation. The RF magnet is normally pulsed for a short time, e.g., milliseconds, under control of the sequencer, and after the pulse, the field exerted by the bias magnet tends to restore the orientation of the magnetic moments of the hydrogen nuclei to the lower energy state.
The magnetic environment of the hydrogen nuclei can be determined by measuring the strength of the magnetic field at the particular location before and after the RF perturbation. The receiver, which may be the same coil as the RF magnet, is used to detect the magnetic field intensity. The information derived from the receiver can be used to determine the magnetic environment through magnetic field intensity measurements. The main discriminatory mechansim currently used is a determination of the density of the nuclei within the scanned location.
Another method of determining the magnetic environment which may become important in the future is the spin-spin relaxation time constant, T.sub.2. Time constant determinations might yield more information than field intensity readings.
Conventional NMR systems, e.g., those not used for biomedical scanning, sample a minute amount of homogenous material. Because of the small size and homogeneity of the material, spatial information concerning the location of the magnetic environment causing the signal is not needed. However, the spatial location of the hydrogen nuclei producing the specific field is of tantamount importance for biomedical applications of NMR. The X, Y, and Z gradient coils of an NMR imaging system are used to perturb the bias magnetic field throughout the sample. A region within the sample, e.g., a point or plane, is left unperturbed by the field gradient. The computer uses the receiver output to identify the type and density of the nuclei in the unperturbed region, thereby identifying the material at that spatial location. The sequencer modifies the pulse duration and strength through the gradient coils as well as the RF magnet which allows the NMR imaging system to scan across the sample or patient.
One form of magnet which can be used as a gradient coil is the so-called "cosine 2.theta." coil. Cosine 2.theta. coils are substantially cylindrical electromagnets having a series of conductors located on the outer surface of the cylinder and extending parallel to the axis of the cylinder. Cosine 2.theta. coils produce magnetic fields perpendicular to the cylinder axis; they cannot be used to produce magnetic fields directed in the axial or Z direction. Since solenoid-type magnets, e.g., electroresistive or superconducting magnets used in conventional NMR imaging systems produce axial fields, cosine 2.theta. coils cannot be used as gradient coils in those systems. The Z-directional gradient on an axial magnetic field (dB.sub.z /dz where B.sub.z is the magnetic field) can be produced by a variable pitch solenoid but it is difficult to produce the desired transverse gradients, dB.sub.z /dx and dB.sub.z /dy, without substantial system modifications.
The bias field produced by a cylindrical permanent magnet formed of an anisotropic magnetic material is a transverse field; that is, the field is constrained to the plane of the cylindrical magnet (by definition the Y-direction) rather than the axial or Z-direction field produced by conventional solenoid magnets. There are now some solenoid magnets which produce this type of transverse bias field. Because of the field direction, cosine 2.theta. coils can be used to produce the requisite gradients on the transverse field in two of the three coordinate directions, specifically the X and Y gradients of the transverse or Y field. However, there are no conventional gradient coils which can produce the requisite field gradient in the axial or Z direction of the transverse (B.sub.y) field.
Accordingly, an object of the invention is to provide a cylindrical electromagnet or gradient coil which produces a transverse field which varies in strength substantially linearly with displacement from a reference point in the direction parallel to the magnet axis. Another object of the invention is to produce a cylindrical electromagnet capable of producing an axial magnetic field which varies proportionally to the distance from the axis of the magnet in a direction parallel to a transverse axis. A further object of the invention is to provide an electromagnet useful as a gradient coil in an NMR imaging system. A still further object of the invention is to provide a cylindrical electromagnet which produces a transverse field with controllable variation parallel to the axis of the magnet. These and other objects and features of the invention will be apparent from the following description and the drawing.